The hearts of bivalved molluscs are structurally and functionally analogous to those of vertebrates, and are favorable model systems for studying the general mechanisms underlying rhythmical activity in cardiac muscle. This investigation of molluscan hearts has been focusing on two agents which profoundly modify rhythmicity: FMRFamide and 5-hydroxytryptamine (5HT). FMRFamide, isolated from extracts of molluscan central ganglia, is the tetrapeptide amide, Phe-Met-Arg-Phe-NH2. It is a cardioexcitor substance initiating, regularizing or augmenting the beat of molluscan hearts and visceral muscle. A radio-immune assay for FMRFamide is now being developed. This technique will be used to determine the taxonomic, histological and subcellular distribution of the peptide in the Phylum Mollusca. The pharmacology of FMRFamide will be tested on three sorts of preparations. Firstly, the effects on the mechanical and electrical activity and on 45Ca ions efflux in a series of bivalve myocardia will be investigated. Secondly, the responses of various homologous visceral muscle preparations representative of other molluscan groups will be examined, with especial attention given to the Cephalopoda. Finally, a representative selection of vertebrate hearts and smooth muscles will be challenged by FMRFamide. An attempt will be made to synthesize and test the effects of analogs. BIBLIOGRAPHIC REFERENCES: Blackwell, J.F., Gainey, L.F., Jr., and Greenberg, M.J. (1977) Shell ultrastructure in two subspecies of the ribbed mussel, Geukensia demissa (Dillwyn, 1817). Biol. Bull. 152:1-11. Reed, C., Greenberg, M.J., and Pierce, S.K., Jr. (1976) The effects of cytochalasins on sponge cell reaggregation: New insights through the scanning electron microscope. In: F.W. Harrison, (ed.), Aspects of Sponge Biology. Academic Press, New York, pp. 153-169.